In drilling wells for oil and gas exploration, understanding the structure and properties of the associated geological formation provides information to aid such exploration. Measurements in a borehole are typically performed to attain this understanding. However, the environment in which the drilling tools operate is at significant distances below the surface and measurements to manage operation of such equipment are made at these locations.
Logging is the process of making measurements via sensors located downhole, which can provide valuable information regarding the formation characteristics. For example, induction logging utilizes electromagnetic signals that can be used to make deep measurements, which are substantially unaffected by the borehole and the effects of the zone invaded by the drilling. Since induction tools may not offer the most reliable measurements in a high resistivity formation, such as a formation having a resistivity greater than hundreds ohm-m, an array laterolog tool may offer more accurate measurements in the high resistivity cases. An array laterolog tool is a current based tool in which a current is generated from the tool and resistivity is determined from measured voltages based on Ohm's law. The array laterolog tool typically includes a central current electrode with additional current electrodes above and below the central current electrode, where the additional current electrodes are used to achieve focusing. Typically, the additional current electrodes can be arranged to force flow perpendicular to the axis of the logging device in a lateral direction. A resistivity log can be made with the tool in an uncased borehole filled with an electrically conductive material. Further, the usefulness of such measurements may be related to the precision or quality of the information derived from such measurements.
Widely used electrical well logging tools have azimuthal symmetrical structures, which may not offer the most accurate formation resistivity in deviated wells, especially in horizontal wells since boundaries and dipping angle can affect responses. Such tools also may not offer the most accurate measurement of the anisotropy of formation resistivity. To more accurately measure formation resistivity in anisotropic formation and deviated wells, tri-axial induction well logging tools have been developed during the past decade. Since induction tools may not offer reliable measurement in high resistivity formation, such as formation resistivity being greater than a hundred ohm-m, array laterolog tool may offer more accurate measurements in the high resistivity cases.
A conventional array laterolog can include a central electrode emitting current, with multiple guard electrodes above and below it such that current is sent between different guard electrodes to achieve greater or less focusing. The larger depth of investigation is provided with greater focusing. Hardware focusing may be further improved by focusing using data manipulation, in which the signals from the measurements are superimposed mathematically to ensure proper focusing in a wide range of conditions.
Some conventional array laterolog tools are operable to generate an average resistivity in deviated wells and horizontal wells. Typically, measurements from these commercial tools are applied to a two-dimensional model used in an inversion scheme to generate formation properties. These conventional tools may be referred to as two-dimensional (2D) tools. The measurements from these tools typically do reflect the formation resistivity when the tool is located in thin layers or is nearby a boundary in thick layers of wells. As a result, it is difficult for log analysts to compute accurate formation resistivity, dip angle in deviated wells, and distance to boundary in horizontal wells using the 2D tools.